The present invention relates to a method for the purification of a fluid and a filtering device with mechanisms for simple maintenance and filter replacement.
Conventional filtering devices essentially comprise a housing with an inlet and outlet for the polluted and the filtered fluid respectively, a filter carrier body which is received displaceably in the housing, and a filter which is located in the filter carrier body. If the filter is clogged, it has to be replaced or cleaned. For this purpose, the filter carrier body is displaced in the housing in such a way that, for example, the inlet of the filter carrier body is closed and the area of the filter carrier body that comprises the filter arrives at a position accessible for cleaning. Afterward, the filter carrier body is displaced again back into its initial position.
Filter replacement systems for plastic melts are described in patent publication EP 0 314 024 A2 and in DE 195 00 060 C, wherein chambers, which each have two mounted filters and are located on a slide, change between positions for filtration and back-flushing of a filter via back-flow channels. Special passages and a long channel are required in the slide in order to be able to react to each position of the slide. This may lead to the formation of dead spaces and more difficult guidance of the slide. In the case where thermally unstable fluids are filtered, decomposition may occur here with the formation of undesired or hazardous decomposition products.
WO 92/16351 describes a simple system for the optional back-flushing of a filter by displacing a filter block between an outlet position and an inlet position. This system lacks a filter replacement position that enables parallel further filtering, as has already been described in EP 0 314 024 A2.
WO 98/22198 A1 shows a filtering system with a housing split up into two parts, wherein the two housing parts are spaced apart from one another in such a way that the filter can be brought by displacement of the piston into a position accessible from the exterior between the housing parts. In this position and also in a back-flushing position, parallel further filtering is however not possible. Between the piston and the two housing parts, spinning compound constantly exits from the housing to the exterior through a total of three annular gaps. According to WO 98/22198 A1, therefore, a flushing flow is not formed between the inlet and outlet, but rather the flushing is brought about by leakage flows proceeding from the inlet flow and from the outlet flow directly to the exterior.
DE 94 18 880 U1 relates to a filtering system with back-flushing, wherein the filter is mounted on a supporting sieve on the appropriate side of the filter for the purpose of back-flushing.
DE 195 19 907 A1 describes a sieve replacement system, wherein two inlet channels each with one sieve converge and are operated either jointly in a flow direction or wherein an inlet can serve as an outlet for the back-flushing.
EP 0 922 558 A1 describes a device for filtering viscous spinning compounds, such as for example NMMO-cellulose solutions, wherein a sieve carrier body is mounted displaceably in a housing, wherein a gap is provided between said carrier body and its guide, which sieve carrier body is held in a floating position by the pressure of the filtered spinning compound without the need for spacers.
A comparable device is also shown in WO 02/16113 A2, wherein two carrier bodies each with two sieve arrangements are provided in the corresponding housing, so that the effective sieve area is reduced by only 25% during the regeneration of just one of the total of four sieve arrangements.
EP 0 781 356 B1 shows a method for transporting a cellulose solution in an aqueous tertiary amine oxide, wherein reliable transport of the solution is achieved by providing an opening at points with low flow rates, from which opening a part of the solution exits without being united again with the main flow. A leakage is consciously accepted in order to improve the flow.
EP 0 915 729 B1 relates to a filtering device with spacers, which are avoided in the aforementioned EP 0 922 558 A1. These two patent specifications have in common the knowledge that many spinning solutions, such as NMMO-cellulose solutions, lead to deposits in the guides of sliding or rotation elements, which form corrosive or other side-products which are detrimental to the product quality. Instead of avoiding dead spaces, gaps are intentionally formed, which are intended to be flushed regularly. A drawback with this device is the occurrence of leakages, i.e. the loss of spinning solution: the fluid flow that passes into the gap is discharged to the exterior.